1. Technical Field of the Invention
The present invention relates to image forming apparatus provided with a plurality of developing units, and more particularly to an image forming apparatus which is designed for selectively using a plurality of developing units supported on a movable support member by moving or rotating the developing units for changeably positioning the developing units at a location of development mainly for use in color copying machines, color printers and the like.
2. Brief Description of Related Art
Japanese Published Patent Application TOKKAI SHO No. 61-151577 discloses an apparatus which is designed for changeably using a plurality of developing units by a movable support member in a rotary method.
Meanwhile, Japanese Published Patent Applications TOKKAI SHO Nos. 57-204566 and TOKKAI SHO 57-204567 disclose apparatuses which are designed for changeably using a plurality of developing units by a movable support member in an elevation method.
Generally, a color copying machine is designed for forming a colored image by using colored developers of four colors. Incidentally, a movable support member provided with a plurality of developing units weighs heavily and it becomes difficult to drive and control the movement of the support member because of its increased resistance for positioning at a predetermined location by successive drive. In order to solve the problem, it is desired to balance the movement without any resistance to whatever position the movable support member is moved.
In a rotary method movable support member, the movement is balanced by disposing a required number of developing units spaced apart on the same circle around the central line of rotation of the movable support member.
In an elevation method movable support member, the movement is balanced by providing a balancer which works with a force almost equivalent to the gross weight of the support member in the direction opposite to the weight of the movable support member.
However, each one of the developing units is temporarily removed for exchanges of developers, sealing materials and bearings or replaced with another developing unit thereby causing in some cases one to forget to mount the developing units. When a developing unit is left unmounted, in case of the rotary method movable support member, the movement is unbalanced with effect of large eccentric gravity when it is driven since the weight balance around the central line of its rotation is collapsed, while in the case of the elevation method movable support member, the movement of the movable support member is unbalanced since the weight balance with the balancer is collapsed.
Furthermore, in the case of the elevation method movable support member in which a spring type balancer is used, the balance of movement of the movable support member is collapsed since the effect of the balancer is gradually weakened because of its fatigue with the passage of time.
In any case, excessive burden is loaded on a driving means in proportion to the operational balance lowered. The movable support member fails to operate or it becomes hard to start operation because of the shortage of torque since the smallest possible capacity of motor is utilized in view of manufacturing cost and advantageous operational control for the driving means. In the worst case, it causes to damage the cogs of a gear.
The Japanese Published Patent Application TOKKAI SHO No. 61-151577 discloses an apparatus which is provided with a sensor fixed on the fitting section of each developing, unit of a movable support member for detecting whether the developing unit is properly mounted or not, by which the problem of leaving a developing unit unmounted can be avoided since the detection by sensors is performed at the fitting sections of each developing unit. However, such detecting method can not deal with the case when the movement of the elevation method movable support member is unbalanced by fatigue of the balancer used with the support member. Furthermore, it invites a rise in manufacturing cost since the sensor has to be provided with each one of the developing units of the movable support member.
There is also a problem that the cord used for connecting the sensor with a device located outside the movable support member tends to be pulled about by the rotation of the movable support member. Especially in the case of the rotary method, the cord can not be pulled about since the movable support member is switched and moved in only one direction. Even in the case of the elevation method, it is not a small burden to pull about the cords of each one of the developing units. In order to deal with the problem, a conductive brush may be considered to be used. However, it causes manufacturing costs to be raised.
The present invention is, therefore, aimed at providing an image forming apparatus capable of solving all of the above-mentioned problems.